Spin transfer (Spin torque) devices are based on a spin-transfer effect that arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current transverses a magnetic multilayer in a CPP (current perpendicular to plane) configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high, and if the dimensions of the multilayer are small. Spin transfer devices also known as spintronic devices wherein at least one of the ferromagnetic layers in a magnetoresistive (MR) junction has perpendicular magnetic anisotropy have an advantage over devices based on in-plane anisotropy in that they can satisfy the thermal stability requirement but also have no limit of cell aspect ratio. As a result, spin valve structures based on PMA are capable of scaling for higher packing density which is a key challenge for future MRAM (Magnetoresistive Random Access Memory) applications and other spintronic devices such as microwave generators.
A spin torque oscillator (STO) is a magneto-resistive (MR) thin film device which can have an induced RF frequency magnetization oscillation within at least one of the magnetic layers of the device by applying an electrical current. As described in U.S. Pat. No. 7,616,412, an STO may be used as a high Q factor RF signal generator if the oscillating magnetization is transformed into resistance fluctuations through a MR effect. In U.S. Patent Application Pub. 2009/0310244, STO is employed as a RF magnetic field generator to assist a magnetic recording process in a magnetic recording device. A STO comprises at least three layers including a magnetic oscillating layer (MOL), a magnetic reference layer (MRL), and a non-magnetic spacer sandwiched between the MOL and MRL. When electrons transit the MRL and become polarized, the polarized electrons then pass through the non-magnetic spacer and through the MOL to induce a gyromagnetic oscillation also known as ferromagnetic resonance (FMR) in the MOL. A PSTO (perpendicular spin torque oscillator) is a version of an STO wherein the MRL has perpendicular magnetic anisotropy (PMA) and is magnetized in a direction perpendicular to planes of the junction layers. With a perpendicular magnetization of the MRL, a full amplitude in-plane oscillation of the MOL can be achieved.
Referring to FIG. 1, a PSTO structure is depicted from U.S. Pat. No. 7,616,412 and includes MRL 1 that has intrinsic anisotropy which keeps its magnetization perpendicular to the film plane, non-magnetic junction layer 2, and MOL 3. Layers 1-3 constitute the PSTO component 11 of the stack. There are other layers 4-8 above the MOL 3 for sensing the magnetization oscillation of the MOL. In particular, second junction layer 4, reference layer 5 with in-plane magnetization, non-magnetic exchange layer 6 typically made of Ru, and pinned layer 7 are sequentially formed on PSTO stack 11. Layers 5-7 comprise a generic synthetic anti-ferromagnetic (SAF) configuration commonly used in commercial giant magnetoresistive (GMR) or tunneling magnetoresistive (TMR) sensors. Anti-ferromagnetic layer 8 pins reference layer 5 and pinned layer 7 through exchange coupling. During an operating mode, electrons flow through the entire stack from top electrical contact 22 to bottom electrical contact 21. As a result of the spin torque effect, electrons passing through the junction layer 2 excite MOL 3 magnetization from a quiescent state into an oscillation state.
PSTO 11 is intrinsically a MR junction in which relative magnetization angle change between layers 1 and 3 will produce a resistance change across stack 11 that can be measured as a voltage signal when a current flows through the stack. However, when MOL 3 reaches a stable magnetic oscillation with a significant amount of in-plane magnetization component, the relative angle between the magnetizations of MRL 1 and MOL 3 does not really change which makes the detection of the actual FMR frequency of MOL 3 hard to achieve when only measuring the resistance between the MRL and MOL. For FMR or RF voltage signal generation purposes, the prior art utilizes SAF and AFM layers above MOL 3 where reference layer 5 serves to generate a MR resistance change during MOL magnetization oscillation. Therefore, layers 3-5 form another MR junction wherein the relative magnetization angle change between MOL 3 and reference layer 5 produces an effective resistance. As current flows between contacts 21 and 22, a voltage signal reflecting MOL layer magnetization oscillation can be produced across the entire stack.
However, there are disadvantages associated with the prior art as pictured in FIG. 1. First of all, the additional layers 4-8 above MOL 3 significantly increase the overall stack thickness and thereby limit its feasibility in thin film applications as in a magnetic recording device. Secondly, the spin torque effect from the top MR junction that is comprised of layers 3-5 for sensing the MOL 3 oscillation interferes with the spin torque effect of the bottom junction of the STO stack 11 so that MOL oscillation quality is degraded by the presence of the SAF layers 5-7 and AFM layer 8.
Referring to FIGS. 2a-2b and FIGS. 3a-3b, micro-magnetic simulations are shown for a PSTO comprised of stack 11 only (FIGS. 2a, 3a) and for a PSTO that includes all layers 1-8 in FIG. 1 as depicted in FIGS. 2b, 3b. FIG. 2a is the oscillation time trace of the in-plane magnetization component for MOL in stack 11 only, and FIG. 2b is a similar oscillation time trace for a PSTO having all layers 1-8. FIG. 3a and FIG. 3b are the corresponding power-spectrum-density (PSD) plots of the time traces in FIGS. 2a, 2b, respectively, where the FMR peak represents the frequency and power of the oscillation. For a top junction, SAF and AFM layers are included in a PSTO stack as represented in FIGS. 2b, 3b, and MOL oscillation shows irregular behavior and lower power than in the case of PSTO stack 11 only. Therefore, the additional layers 4-8 are useful in acquiring oscillation information of the MOL 3, but the second (top) MR junction also changes the MOL oscillation behavior. The change to irregular oscillations and lower power is especially not desirable when uniform and high power MOL oscillation is required on a continuous basis for optimum performance while characterization of oscillation frequency is only needed occasionally, for example, when the PSTO is used as a RF field generator in a magnetic recording device.
For read out of PSTO MOL oscillation frequency through PSTO resistance, the prior art also describes using a MRL with vertical and in-plane anisotropy to achieve a tilted MRL magnetization as shown in FIGS. 1c, 1d of U.S. Pat. No. 7,616,412. In-plane anisotropy keeps the MRL magnetization at an angle less than 90 degrees to the film plane to provide a reference magnetization direction for MOL oscillating magnetization. However, this approach requires development of two anisotropy axes in the magnetic MRL film, and a MRL film structure that is more complicated than needed in situations where a simple PSTO structure with perpendicular MRL is desired as in examples where a PSTO serves as a RF magnetic field generator in a magnetic recording device. Furthermore, in-plane anisotropy as an intrinsic property of the MRL remains present even when the MOL frequency of a PSTO is not being characterized.
In U.S. Pat. No. 7,471,491, an external in-plane field is used to shift an existing FMR mode from a low frequency to a higher frequency and the amount of the shift is related to in-plane field magnitude that a sensor experiences. As a result, the prior art uses this scheme to read out the magnetic bits from a magnetic recording medium. Output is measured through a system consisting of a waveguide connected through a probe to a bias and amplifier and then to a noise spectrum analyzer.
U.S. Patent App. Publication 2009/0201614 discloses a hybrid spin torque oscillator having a separate oscillating field generating unit that supplies an oscillating field through magnetostatic coupling to a magnetoresistive (MR) element. When a DC current is applied to the MR element in the presence of the oscillating field, magnetic resonance occurs in the MR free layer. An AC component is formed by device resistance variation as a function of time and is extracted by a bias tee formed with a capacitor and an inductor to obtain a microwave output.
U.S. Pat. No. 7,652,915 describes a method for measuring resonant frequency in a MR element after a DC current is applied.
U.S. patent app. Publication discloses a spin torque oscillator without an additional MR element but a method of measuring oscillating frequency is not discussed.